Coordinate measuring machines (CMMs) are often used for measurements on workpiece surfaces. A coordinate measuring machine usually includes a table that carries the workpiece to be measured, a probe or some other measuring instrument that is positioned directly above the workpiece, and a measuring head, which exerts defined actuating forces on the measuring instrument and measures forces transmitted from the measuring instrument to the measuring head. In general, coordinate measuring machines additionally include a movement device that moves the measuring head in three orthogonal movement directions x, y, and z relative to the table with high accuracy. However, coordinate measuring machines having a movement table that moves relative to a stationary measuring head are known. Insofar as reference is made hereinafter to a movement device, the relevant observations are correspondingly applicable to movement tables.
A coordinate measuring machine additionally includes an evaluation and control device, which controls the movements of the movement device and evaluates the measurement signals generated by the measuring head and possibly a measuring instrument secured thereto. For each of the three movement directions x, y, and z, the movement device has at least one transducer which returns information about the travel distances covered to the evaluation and control device. As a result, the position of a coupling of the movement device to which the measuring head is secured in an exchangeable fashion is known with high accuracy in all movement positions.
If the surface information to be determined by measurement includes in the Cartesian coordinates the workpiece surface, then the measuring instrument is usually a tactile probe. The latter, during the measurement, touches the surface with a predefined probing force generated by the measuring head. During probing, the probe is slightly deflected, which is likewise detected by the measuring head. If the position of the probe with respect to the coupling of the movement device is known, the Cartesian coordinates of the contact point can be accurately determined when the probing element contacts the workpiece surface. Instead of a tactile probe, it is also possible to use an optical probe, which measures the distance to the workpiece surface without contact. Such optical probes are based, e.g., on the principle of chromatic confocal imaging and are expedient primarily for the measurement of very soft workpieces.
If the roughness of workpiece surfaces is intended to be measured, the movement device of the coordinate measuring machine bears a roughness sensor, the latter often being a stylus instrument. A stylus instrument includes a movably mounted measuring arm, to the end of which a probe element, e.g., a diamond tip, is secured, which is deflected during the measurement by the contact with the workpiece surface. During the measurement, the probe element is linearly moved perpendicularly to the deflection direction of the probe element with the aid of a feed unit and in this way is guided along a line over the workpiece surface to be measured. Accurate measurement values can be obtained only if the deflection direction of the probe element extends exactly perpendicularly to the surface to be measured. Therefore, the probe element has to be aligned very accurately relative to the workpiece not only with regard to its Cartesian coordinates, but also with regard to its angular orientation in space.
The same correspondingly also applies to roughness sensors that operate without contact, for instance white light sensors that carry out point measurement or areal measurement. Sensors of this kind have also to be aligned very accurately relative to the workpiece to prevent the measurement results from being corrupted.
In modern production sequences, the workpieces have to be manufactured with such small tolerances that continuous process monitoring is indispensable. Moreover, a problem arising ever more frequently is that the workpieces whose surfaces are intended to be measured in an automated fashion have very complex shapes. By way of example, an engine block of an internal combustion engine has a multiplicity of bores with different internal diameters, numerous undercuts and irregular recesses at which there are surfaces to be measured. Conventional coordinate measuring machines with their usually very high-volume movement devices are generally unable to position a measuring instrument in the openings or recesses of an engine block in such a way that a measurement can be performed there.
Modern measuring systems which in some instances are also suitable for continuous process monitoring therefore often include a positioning apparatus arranged between the movement device of the coordinate measuring machine and a probe, a roughness sensor or some other measuring instrument. The positioning apparatus has the task of positioning the measuring instrument directly above the surface to be measured.
Such positioning apparatuses can include, for example, a rotary-pivoting joint, as described in U.S. Patent Application Publication No. 2009/0255139 A2. With the aid of this known positioning apparatus, a measuring instrument can be rotated about a vertical axis and additionally pivoted about a horizontal axis to position the measuring instrument optimally relative to the workpiece surface. In that case the measuring instrument is additionally also rotatable about a third axis of rotation.
Another positioning apparatus having three axes of rotation is known from DE 20 2014 101 900 U1.
In general, the positioning apparatus is not secured directly to the movement device of the coordinate measuring machine, but rather to the measuring head carried by the movement device. However, measuring systems in which the measuring head is arranged between the positioning device and the measuring instrument are also known. The measuring instrument is not secured to the positioning device directly, but rather indirectly via the measuring head.
Positioning apparatuses of this type have to position the measuring instrument very accurately. An inaccurate positioning of the measuring instrument carried by the positioning apparatus is manifested directly in a lower measurement accuracy of the entire measuring system. In order to achieve a high positioning accuracy, both the mechanical construction and the control of the drives of the positioning apparatus have to meet very stringent requirements.
In positioning apparatuses of this type, an electrical servomotor is usually used for each degree of freedom of movement. The servomotor together with a displacement sensor or rotary encoder are part of a closed-loop control circuit. By comparing the controlled variable determined by the sensor or encoder with the reference variable (setpoint value), the controller calculates the current that is intended to be applied to the servomotor. Besides a position sensor or angle encoder, the closed-loop control circuit normally contains an additional tachometer, which measures the (rotational) speed of one of the parts. In this way, speeds can be determined more rapidly than as it would be possible by differentiating the position information with respect to time.
In general, the closed-loop control has the effect that current is applied to the servomotor even when the parts to be driven are not moving. In this way, the servomotor generates holding forces required to keep the relevant parts of the positioning apparatus stationary.
What is disadvantageous about this type of closed-loop control, however, is that electric current flows even in the quiescent state. The electric current results in local heating of the positioning apparatus in the vicinity of the respective servomotor. This necessitates additional measures to compensate the resulting thermally governed expansion of parts of the positioning apparatus or to take the resulting thermally governed expansion of the parts of the positioning apparatus into account computationally.
U.S. Pat. No. 8,001,859 B2 discloses a positioning apparatus which, in order to solve this problem, contains a mechanical friction brake that locks the parts of the positioning apparatus that are movable relative to one another as soon as the parts are intended to no longer move. The servomotors can then be deenergized, as a result of which less heat is generated.
What is disadvantageous about this known positioning apparatus, however, is that it can assume two different operating states depending on whether the brake was actuated. As a result, two calibration processes are necessary in order to calibrate the measuring system. Moreover, such a brake requires an additional actuator mechanism and corresponding control lines, thereby occupying additional structural space. What is of importance for the positioning apparatuses, however, is that they are as small and lightweight as possible, to be able to introduce the measuring instrument or the measuring head even into narrow openings or other poorly accessible locations.